Collections of serous fluid containing bioactive substances, such as interferon, antibodies, hormones, etc., are conventionally carried out first by venopuncture with a large size needle to draw the fresh whole blood. The procedure involves discomfort to the subject, and risk of introducing infective organisms into the blood stream which circulates the whole body. Further, the cellular and coagulative components of the blood sample obtained must be separated to yield the serum part which theoretically amounts to about 50% of the original blood volume. However, in practice, the yield is often less due to entrapment in the gel formed by the blood clot. Therefore, an implant which allows the penetration of serous fluid for collection, when required, will be very desirable.
An implant is also needed to deliver medicament that may be required on a sustained or intermittent basis. These implantable drug delivery devices come in many forms, designs, sizes and principles of operation. Generally, they can be classified into 2 categories. The physical pumps deliver a drug solution by electrically propelled roller pumps or by a collapsible bellow compressed under the pressure of a volatile liquid. These pumps are bulky, and have a maximum drug reservoir capacity of about 45 ml which must be refilled frequently. In addition, only drugs that are soluble and stable in solution can be infused. In the other category are the smaller diffusional implants which mainly operate on 2 design variations. The first one is to disperse a small amount of the active agent in a solid excipient mass which is either inert or hydrolyzable by body fluid to facilitate the outward diffusion, especially of drugs with molecular weight &gt;1,000 daltons. The other design is to put a drug solution in a flexible compartment with an exit orifice. A part of this compartment is surrounded by an osmotic agent which is enclosed further by a rigid, but permeable capsule. The body fluid imbibed into the capsule due to the osmotic agent compresses the drug compartment and expels the drug solution through the orifice. However, these diffusional implants are not readily amenable to the periodic augmentation in drug doses often required, e.g., in the treatment of diabetes by insulin, or to interruption of medicament, e.g., in contraception which requires steroid hormone daily for 21 days in a month.
In consideration of the aforementioned, a novel implantable device should be small for easy insertion, and should allow the penetration of serous fluid for sampling and harvesting of bioactive materials, or dissolution of a drug, if included therein, for dispensing on demand. If required, replenishment of the medicament should be readily implemented to last for a reasonable period of time. Further, when used as a drug delivery device, it should be able to discharge a drug in small amount, but if required, an increase in dose can also be implemented.